Can Google’s rivals team up to take a chunk out of its ad business? Microsoft, Yahoo, and AOL are planning to work together to sell ads on each other’s sites, according to The Wall Street Journal. The companies would basically create a single ad platform across the sites, which would simplify the work of advertisers who generally have to negotiate the three company’s separate systems. (Each site will continue selling ads individually as well.) The deal, if it is made, is likely to face regulatory scrutiny.